2. Field of the Invention
This invention relates to a floating device resting on a recording or reproducing unit in a magnetic disc device or an optical disc device.
2. Description of the Prior Art
Floating devices of the dynamic pressure type have heretofore been most popular for application to the floating magnetic heads in the magnetic disc devices for electronic computers or in the video discs for the recording of still TV images. p In such a floating device of the dynamic pressure type, all the buoyancy for floating the device is created by a floating shoe. More specifically, the buoyancy is created, during high-speed rotation of the disc, by the dynamic pressure of gas flowing into between the surfaces of the floating shoe and the disc due to the curvature of the floating shoe surface, and the buoyancy is variable in proportion to the relative velocity of the shoe and the disc. Such a floating device is disclosed in an article entitled "A GAS FILM LUBRICATION STUDY" in IBM Journal, July, 1959.
The dynamic pressure type floating device causes the floating shoe to be floated up a minute distance from the disc, with the disc surface as the floating surface, by imparting a predetermined pressure force from a spring such that an air stream created over the disc surface by rotation of the disc is opposite in direction to the buoyancy acting on the floating shoe. The amount of such floatation is extremely small and usually of the order of 0.1 to several microns.
Since, as described, the amount of floatation of the dynamic pressure floating shoe floating over the disc is proportional to the relative velocity of the disc and the floating shoe, the floating shoe may touch the disc during the rest condition of the latter or when said relative velocity is low as at the starting or stoppage of the disc rotation, and this may impart damages to the disc or the floating element. To prevent this, a protecting mechanism has heretofore been provided for mechanically separating the floating shoe from the disc.
The protecting mechanism heretofore used has comprised a spring having one end attached to a fixed member, and the floating element including the floating show has been fixed to the other end of the spring. When the number of revolutions of the disc has attained a predetermined value, a plunger or the like pushes the spring to urge the floating element toward the disc, and under the other conditions, the pressure of the plunger is released to permit the floating element to be separated from the disc by the action of the spring.
In such protecting mechanism, however, the pressure spring with the floating element secured thereto is pushed toward the disc to move the floating element, and this has not only limited the stroke of the floating element but also made it difficult to provide any desired stroke. Therefore, the spacing between the disc and the floating element when they are separated must be maintained constant and accordingly, the thickness of the disc resting on the turn table must be constant. Any variation in the thickness of the disc or the configuration of the floating shoe forming the floating element would vary the pressure force taken by the floating element from the pressure spring during floatation, thus varying the amount of floatation of the floating element. Therefore, in the floating device equipped with such conventional protecting mechanism, it has been required that the disc used have a predetermined thickness, and this has greatly prevented wider applications of such devices.
Further, it is usually the case with the dynamic pressure type floating device that when the floating shoe floats above the rotating circular disc, the relative velocity of the disc and the floating shoe and accordingly the buoyancy of the floating shoe is reduced as the shoe is moved radially inwardly of the disc, unless the velocity of the disc rotation is varied. The floating distance of the floating shoe is determined by the balance between the buoyancy and the resilient pressure force with which the pressure spring urges the floating shoe toward the disc surface, and in order to provide a predetermined amount of floatation irrespective of the relative velocity of the disc and the floating shoe, it is necessary to vary the pressure force of the spring in correspondence with the variation in the relative velocity, i.e. the position of the floating shoe in the radial direction of the disc.
In contrast with the above-described dynamic pressure type floating device, there are floating devices called the static pressure type. In such type of device, the buoyancy is provided not by the disc rotation but by forming an air cushion between the floating shoe and the disc with the aid of compressed air injected through a nozzle formed centrally of the floating shoe, and the amount of the buoyancy depends on the balance between the buoyancy and the pressure force with which the spring urges the floating shoe toward the disc surface, and not on the relative velocity of the floatng shoe and the disc.
Again in such static pressure type floating device, the disc surface and the floating shoe must be separated from one another during the rest condition of the device or when the pressure of compressed air is too low to provide a sufficient buoyancy, and it is necessary to provide a protecting mechanism similar to that used with the dynamic pressure type floating device. In this case agan, a predetermined thickness is required for the disc resting on the turn table and no arbitrary thickness is available for the disc.